Hi, although a master gardner with my own home based plant nursery, I rarely use commercially produced 10-10-10, which stands for 10 parts nitrogen, 10 parts potassium and 10 parts potash ( NPK ); but, years ago, I bought several bags of this for the yard in which we'd just planted grass seed the year before. My spreader quit and rather than spread it unevenly by hand, I didn't use the last 2 bags of fertilizer. It stayed under the outside shelter, protected from rain, but we have high humidity here in eastern North Carolina.  The next year when I later had a use for it, the moisture that was absorbed through the bag was enough to cause it to almost harden to the point that I had to drop it on concrete to break it up; although some of it was more like goo than loose fertilizer, I was able to use most of the fertilizer that was broken back into small fragments similar to what it would have been when fresh. Even the gooey fertilizer remaining I used by making a slurry type mixture and applying it in a diluted form around the drip line of several large river birch trees. I've never had any experience with bagged fertilizer beyond 2 years because of the humidity here; but, you can always experiment with older fertilizer on older, misc. plants you wouldn't loose much sleep over if they were damaged or died from the wrong concentration of fertilizer caused from this sitting up period. There is one general rule of thumb I tend to recommend though and it is: "if in doubt, dispose of it." If you had a lot to dispose of, I'd call your county extension agency and ask they're advice on how to safely dispose of it. Around here, they'll occasionally have pickup points for pesticides, they're containers and misc. things you shouldn't just throw into the trash or landfills. Hope this helps. Mark Savage, Lumberton, NC%Savage Gardens.